1. Field of the Invention
The present invention relates to novel aqueous, moisture-resistant, low temperature-curable ceramic slurry compositions which can be sprayed or coated onto ceramic or metallic substrates for thermal protection, insulation, wear-resistance or electromagnetic performance, such as repair compositions for application to metallic exhaust ducts of jet engines.
2. Description of the Prior Art
A wide variety of protective ceramic compositions are known for application to substrates exposed to high temperatures but such known compositions have disadvantages which preclude their use as repair materials, such as very high curing temperatures, and which result in a loss of physical integrity when exposed to humid environments after exposure to temperatures of about 1000.degree. F. Many substrates are made of materials, such as titanium, which cannot withstand the high curing or firing temperatures required for most known ceramic compositions. Also, repair compositions must be applied to installed substrates, and high firing temperatures cannot be used to cure repair ceramic compositions on most such installed substrates, including automobiles and fueled aircraft
Reference is made to U.S. Pat. No. 5,034,358 which discloses a protective zirconia/potassium silicate ceramic composition which is applicable to low melting temperature substrates, such as aluminum, and fired or cured at temperatures not exceeding about 500.degree. F. to provide a ceramic which is wear-, corrosion- and heat-resistant. However such ceramic coatings soften and fail when exposed for a few days to 95% relative humidity at 95.degree. F., which is true of all known sodium and potassium silicate ceramics.
Reference is also made to U.S. Pat. No. 5,310,422 which discloses a barium titanate/potassium/sodium silicate ceramic heat-resistant paint composition, which can contain glass frit instead of the alkali metal silicate, and is applied to substrates as an aqueous slurry and dried to form temperature-resistant paint coatings.
Such paint compositions also soften and degrade in humid environments.
Reference is made to U.S. Pat. No. 5,468,290 which discloses another aqueous ceramic coating composition containing a major amount of a metallic oxide, such as magnesium oxide, an alkali metal silicate, such as a sodium and potassium silicate, and silicon carbide fibers for producing temperature-and abrasion-resistant adhesive layers on sensors and sensor wires mounted on steel internal engine parts and cured at temperatures of 260.degree. C. Such adhesive ceramic layers also soften and degrade in humid environments.
Therefore, there is a need for inexpensive protective ceramic compositions which can be applied by conventional methods, such as thermal spray, sol-gel and wet spray processes, onto a wide variety of substrates, including temperature sensitive substrates, such as titanium, and in temperature-sensitive environments, such as repair materials on installed substrates in an environment where high temperatures are too dangerous and/or are destructive of electrical or other components within said environment, and which are rapidly curable at low temperatures below about 500.degree. F. to provide solid ceramic protective coatings which are resistant to softening and degradation after high temperature use upon exposure to high humidity above about 90% relative humidity, at temperatures above about 90.degree. F. for at least about 7 days.